Spontaneous damage of DNA bases is a major source of cancer-causing mutations. Given the thousands of lesions generated per genome every day, it is remarkable that cancer remains a relatively infrequent event with the majority of cases arising relatively late in life. With increasing life expectancy and exposure to exogenous DNA damaging agents, society bears the ever increasing cost of diagnosing and treating cancer. At the cellular level the ability to safeguard against these spontaneous lesions relies largely on the base excision repair (BER) pathway whereby DNA glycosylases scan the genome to locate and excise base lesions. The action of an apurinic (AP)-specific endonuclease, AP-lyase/DNA polymerase, and DNA ligase are required to complete repair of the DNA. Our long-term goals are to understand how BER proteins locate and selectively act on a wide range of DNA lesions within genomic DNA and how the dynamics of protein-protein and protein-DNA interactions enable coordination of multi-step, multi-enzyme repair pathways. Recent evidence suggests that nucleotide flipping, the process by which a nucleotide is extracted from the DNA duplex and bound in an active site pocket, provides much of the selectivity in distinguishing damaged and undamaged bases. We propose to test this hypothesis by directly observing flipping of damaged and undamaged nucleotides by DNA glycosylases (Aim 1). The genomic search for rare lesions is facilitated by the examination of many nucleotides with each DNA binding event, therefore we will characterize the ability of BER enzymes to move along DNA and measure the efficiency with which sites of damage are productively engaged during a scanning encounter (Aim 2). As DNA repair intermediates are potentially cytotoxic or mutagenic, it is critical that initiated BER events be completed. We propose to investigate the dynamics of protein-protein interactions in BER and determine their functional significance in the coordination of multiple enzymatic activities (Aim 3). By combining the results from pre-steady state enzyme kinetics, fluorescence spectroscopy, and structure-activity relationships we have a unique opportunity to dissect the protein-DNA dynamics important for damage recognition and repair. As BER is a critical component of the cellular defense against cancer, and because these pathways are antagonistic toward some DNA damaging agents used in the treatment of cancer, these studies have the potential to contribute both to our understanding of mutagenesis and to advances in cancer therapy.